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The hypothalamic-pituitary-adrenal (HPA) axis responds to stress by releasing cortisol, which mobilizes glucose and fatty acids, suppresses immunity, and enhances blood pressure. Acute stress responses are adaptive; chronic stress impairs immune function and increases infection risk. The HPA axis exhibits diurnal variation and negative feedback, which can be disrupted in chronic stress or depression.
You already understand from your study of the anterior pituitary that hormonal axes follow a hierarchical pattern: the hypothalamus releases a tropic hormone, the pituitary amplifies the signal, and the target gland produces the final effector hormone. The HPA axis follows exactly this template. When the brain perceives stress — whether physical danger, illness, or psychological pressure — the hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH), which in turn drives the adrenal cortex to synthesize and release cortisol. Cortisol then feeds back to both the hypothalamus and pituitary to suppress further CRH and ACTH release, completing the negative feedback loop.
Cortisol's effects make biological sense when you think of them as preparing the body for sustained challenge. It promotes gluconeogenesis in the liver, breaking down amino acids and glycerol to produce glucose, ensuring the brain and muscles have fuel. It mobilizes fatty acids from adipose tissue for alternative energy. It suppresses non-essential functions that consume resources: immune responses are dampened, inflammatory pathways are inhibited, and reproductive hormone secretion decreases. It also sensitizes blood vessels to catecholamines like epinephrine, helping maintain blood pressure. In the acute setting — an infection, an injury, a threat — these responses are lifesaving. They redirect the body's resources toward immediate survival.
The HPA axis also has a built-in daily rhythm independent of stress. Cortisol follows a diurnal pattern, peaking in the early morning (around 6–8 AM) to prepare the body for waking activity and falling to its lowest levels around midnight. This rhythm is driven by the suprachiasmatic nucleus and is important clinically: a single cortisol measurement means little without knowing when it was taken. Morning cortisol should be high; evening cortisol should be low. Loss of this diurnal variation is itself a sign of HPA axis dysfunction.
The critical distinction is between acute and chronic activation. A brief cortisol surge during an exam or a near-miss in traffic is adaptive — it enhances alertness, mobilizes energy, and resolves within hours. But when stress is unrelenting — chronic work pressure, ongoing illness, prolonged psychological distress — the HPA axis can become dysregulated. Cortisol levels may remain persistently elevated, or the normal feedback mechanisms may become blunted so that the axis no longer responds appropriately. The consequences of chronic hypercortisolism are essentially the acute effects taken to a pathological extreme: sustained immunosuppression increases susceptibility to infection, persistent gluconeogenesis contributes to hyperglycemia and muscle wasting, and chronic vasoconstriction promotes hypertension. This is why understanding the HPA axis is central to both endocrinology and the physiology of stress-related disease.